1. Field of Invention
The present invention relates generally to laser ablation of conductive films and, more specifically, to a method for laser patterning a multilayered conductor/plastic substrate structure and to multilayered electrode/plastic substrate structures and display devices incorporating the same.
2. Description of the Related Art
A liquid crystal display (LCD) is a type of flat panel display used in various electronic devices. Generally, LCDs comprise two sheets of polarizing material with a liquid crystal solution therebetween. Each sheet of polarizing material typically comprises a substrate of glass or transparent plastic; the liquid crystal (LC) is used as optical switches. The substrates are usually manufactured with transparent electrodes, typically made of indium tin oxide (ITO), to which electrical “driving” signals are coupled. The driving signals induce an electric field which can cause a phase change or state change in the LC material; the LC exhibiting different light-reflecting characteristics according to its phase and/or state.
Liquid crystals may be nematic, smectic or cholesteric depending upon the arrangement of the molecules. A twisted nematic cell is made up of: two bounding plates (usually glass slides), each with a transparent conductive coating (such as ITO) that acts as an electrode, spacers to control the cell gap, two crossed polarizers (the polarizer and the analyzer), and nematic liquid crystal material. Twisted nematic displays rotate the director of the liquid crystal by 90°. Super-twisted nematic displays employ up to a 270° rotation. This extra rotation gives the crystal a much steeper voltage-brightness response curve and also widens the angle at which the display can be viewed before losing much contrast. Cholesteric liquid crystal (CLC) displays are normally reflective (meaning no backlight is needed) and can function without the use of polarizing films or a color filter. “Cholesteric” means a type of liquid crystal having finer pitch than that of twisted nematic and super twisted nematic. Sometimes it is called “chiral nematic” because cholesteric liquid crystal is normally obtained by addling chiral agents to host nematic liquid crystals. Cholesteric liquid crystals may be used to provide bi-stable and multi-stable displays that, due to their non-volatile “memory” characteristic, do not require a continuous driving circuit to maintain a display image, thereby significantly reducing power consumption. Ferroelectric liquid crystals (FLCs) use liquid crystal substances that have chiral molecules in a smectic C type of arrangement because the spiral nature of these molecules allows the microsecond switching response time that make FLCs particularly suited to advanced displays. Surface-stabilized ferroelectric liquid crystals (SSFLCs) apply controlled pressure through the use of a glass plate, suppressing the spiral of the molecules to make the switching even more rapid.
Some known LCD devices include chemically-etched, transparent, conductive layers overlying a glass substrate. See, e.g., U.S. Pat. No. 5,667,853 to Fukuyoshi et al., incorporated herein by reference. Unfortunately, chemical etching processes are often difficult to control especially for plastic films. As a consequence, electrodes resulting from such processes are often misshaped, with “wells” being formed near the substrate in instances where too much etchant was employed. Moreover, the minimum line gaps obtained in plastic films are typically limited to 15 μm or more. Additionally, concerns for the environment lessen the desirability of employing chemical etching processes which produce dangerous and/or harmful byproducts.
There are alternative display technologies to LCD's that may be used for example in flat panel displays. A notable example is organic light emitting devices (OLEDs), which are comprised of several layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device. An OLED device is typically a laminate formed on a substrate such as glass. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between an anode and a cathode. The semiconductor layers may be hole-injecting and electron-injecting layers. The light-emitting layers may be selected from any of a multitude of light emitting organic solids, e.g. polymers. When a potential difference is applied across the cathode and anode, electrodes from the electrode-injecting layer and holes from the hole-injecting layer are injected into the light-emitting layer. They recombine, emitting light.
In a typical matrix-addressed light-emitting display device, numerous light emitting devices are formed on a single substrate and arranged in groups in a regular grid pattern. Activation may be by rows and columns, or in an active matrix with individual cathode and anode pads. OLED's are often manufactured by first depositing a transparent electrode on the substrate, and patterning the same into electrode portions. The organic layer(s) is then deposited over the transparent electrodes. A metallic electrode may be formed over the electrode layers. For example, in U.S. Pat. No. 5,703,436 to Forrest et al., incorporated herein by reference, transparent indium tin oxide (ITO) is used as the hole-injecting electrode, and a Mg-Ag-ITO electrode layer is used for electron injection.
An excimer laser has been employed to pattern ITO electrode material overlying a glass or quartz substrate. See, e.g., U.S. Pat. No. 4,970,366 to Imatou et al. and European Patent Specification EP 0 699 375 B1 by Philips Electronics N.V., both incorporated herein by reference. However, electrode/substrate structures formed with glass or quartz substrates lack the flexibility and thickness desired for many display products.
F. E. Doany et al., “Large-field scanning laser ablation system”, IBM Journal of Research and Development, Vol. 41, No. ½, 1997, incorporated herein by reference, discloses a large-field scanning laser ablation system which employs a XeCl 308 nm excimer laser and a mask for ablating vias (down to 8 μm) in a polyimide layer. The system employs a projection lens (Dyson-type) to image a portion of a full-field mask onto a portion of the substrate. The system also includes a light tunnel/homogenizer which outputs a square beam with uniformity of ±5% across the entire output field, producing an 8-mm×8-mm illumination spot at approximately 0.05 NA.
Excimer lasers have also been used to manufacture thin-film transistors (TFTs). For example, “Flat-Panel Displays Slim Down with Plastic”, Science and Technology Review, Nov. 1999. incorporated herein by reference, discloses using an excimer laser to manufacture TFTs on top of thin, plastic sheets. In this reference, an amorphous silicon dioxide layer acts as a thermal barrier to prevent the plastic (PET) substrate from heating and melting. See also, U.S. Pat. No. 5,714,404 to Mitlitsky et al. and U.S. Pat. Nos. 5,817,550 and 5,856,858 to Carey et al., both incorporated herein by reference, which disclose using an excimer laser for crystallizing a TFT silicon layer and for doping.
It is also known to employ an infra-red (IR) fiber laser for patterning a metallic conductive layer overlying a plastic film, directly ablating the conductive layer by scanning a pattern over the conductor/film structure. See: Int. Publ. No. WO 99/36261 by Polaroid Corporation; and Chu et al., “42.2: A New Conductor Structure for Plastic LCD Applications Utilizing ‘All Dry’ Digital Laser Patterning,” 1998 SID International Symposium Digest of Technical Papers, Anaheim, Calif., May 17-22, 1998, no. VOL. 29, May 17, 1998, pages 1099-1101, both incorporated herein by reference. However, metallic conductive layers formed from silver-based, transparent, conductor materials are relatively expensive. Moreover, employing the aforementioned direct lasering techniques is relatively slow and requires complex laser control mechanisms and algorithms to control and direct the narrow IR laser beam.
Accordingly, a high-speed, high-precision, chemical-free method for patterning conductor/plastic substrate structures is needed. To this end, it would also be desirable to have available a flexible conductor/plastic substrate structure with a “glass replacement” structure which includes material insulating the glass replacement structure from heat generated during laser irradiation of the conductor/substrate structure. A method for patterning conductor/substrate structures which is sufficiently fast to accommodate a roll-to-roll manufacturing process “downstream” of the patterning process would also be useful and potentially yield cost savings in the manufacturing of LCD devices.